Studies were undertaken to elucidate whether orphenadrine influences thyroid function. Seven volunteers were given orphenadrine in weekly increasing dosage up to 300 mg per day; in 5 patients chronically treated with 300 mg orphenadrine daily the drug was gradually discontinued. No changes were found in PBI, RT3U, TT3 and TSH during or after orphenadrine medication; also TSH- and TT3-responses to 200 microgram TRH iv were not influenced by the drug. Orphenadrine medication increased serum thyroxine values (P less than 0.001) as measured with the competitive protein binding (CPB) technique, but did not influence serum thyroxine values measured by radioimmunoassay. Orphenadrine added to serum in vitro in the Murphy-Pattee assay did not increase thyroxine values; two out of eight tested metabolites however did. It is concluded that orphenadrine in a dosage up to 300 mg per day does not influence thyroid function. It increases serum thyroxine levels as measured by the competitive protein binding technique of Murphy and Pattee. This is due to an in vitro competition between ethanol-extractable orphenadrine metabolites and thyroxine for binding sites on the thyroxine binding globulin.